A constant-on-time (COT) regulator, or power converter, generally produces an output voltage from an input voltage through a power switch, such as a transistor. The power switch is typically controlled by operation of a feedback comparator and an on-time circuit, which cause the power switch to be turned on and off. The on-time for the power switch is generally held constant by the on-time circuit, during which time an output inductor and output capacitor are charged up by the input voltage to provide power to a load, and the output voltage increases. At the end of the on-time, the power switch is turned off, and the output voltage decreases. The off-time for the power switch may vary, depending on how quickly a feedback voltage (based on the decreasing output voltage) drops below a reference voltage, as determined by the feedback comparator. When the feedback comparator determines that the feedback voltage has fallen below the reference voltage, the power switch is turned back on for the duration of the on-time, and this on-off cycle repeats.
The on-off cycle results in the output voltage having a frequency component. During steady state operation, i.e., when the load does not vary significantly, the on-off cycle exhibits a relatively steady switching frequency. The switching frequency of the output voltage can cause undesirable harmonics in other components of an overall electronic circuitry due to electromagnetic interference. For some types of electronic circuitry, therefore, it is desirable to ensure that the switching frequency of the output voltage is maintained at a known desired value that will not unduly affect the other components in the overall circuitry.
The switching frequency is dependent on several factors, including the duration of the on-time. It is, thus, possible to ensure a certain frequency during steady state operation by adjusting the on-time. Some other factors that can affect the frequency are caused by various component parameters that result in propagation delays, deadtimes, and losses within some of the components of the power converter. Some of these factors are not very significant when the on-time is relatively long, in which case, a common relationship between the frequency and the on-time provides for a general design formula for determining the appropriate component values for the power converter circuitry. However, when the on-time is relatively short, e.g., approaching 100 ns or less, the general design formula does not apply, due to a greater relative effect of the various propagation delays, deadtimes, and losses within some of the components. In other words, at a relatively short on-time (i.e., a low duty cycle with a relatively high switching frequency), the typical circuit design can result in a switching frequency that is substantially different from the desired value, such that there is a higher risk that it can unduly affect the other components in the overall circuitry.
Additionally, when a transient occurs (e.g., a sudden and significant change in either the load or the input voltage), the switching frequency can undergo substantial instability, or fluctuations, before settling back into steady state operation. Although this instability is temporary, it is desirable for the switching frequency to settle into steady state operation relatively quickly.